In early modern Europe, Satan's horns would have been a sign that his missus was doing the dirty on him.

I wonder how the cuckold imagery came to overlap with the Devil imagery -- I think the latter is a fairly recent development, though as a good Jungian, I am bound to say it sprang somehow from a much older archetype.

Jon Pertwee's Doctor makes this very point in The Daemons, my and his favourite Who story ever. (Horns are associated with power because of a race of horned aliens who come and poke their noses in our affairs every so often.)